Write the various mechanisms of absorption of compounds.

1. Absorption of part of glucose, amino acids and some electrolytes like chloride ions are absorbed by simple diffusion depending on concentration gradient. 

2. Some amino acids as well as substances like fructose are absorbed by facilitated transport. 

3. In this method, carrier ions like Na+ bring about absorption. 

4. Some ions are absorbed against concentration gradient. It requires energy. This type of absorption of mineral like sodium is called active transport. 

5. Water is absorbed along the concentration gradient. 

[Note: Glucose and galactose are transported into absorptive cells of the villi through secondary active transport that is coupled to the active transport of Na . Amino acids are transported via active transport.]